Method of rectifying a neutral molten salt treating bath



Patented June 28, 1949 METHOD OF RECTIFYING A NEUTRAL MOLTEN SALT TREATING BATH Paul H. Kramer, Detroit, Mich., assignor to Park Chemical Company, Detroit, Mich., a corporation of Michigan No Drawing. Application March 23, 1945, Serial No. 584,490

2 Claims. 1

The present invention relates to the heat treating of steel in a neutral molten salt bath, and particularly to a method of maintaining such baths in a neutral condition to avoid decarburization of the steel.

In the heat treatment of steel, and particularly the hardening of the medium and high carbon steels, it is particularly important that no decarburization result. For this reason neutral molten salt baths are widely used for such heat treating operations, and a number of different types give satisfactory results. Among them are barium chloride baths and baths comprising mixtures of sodium and potassium chlorides.

While these neutral salt baths give satisfactory results when made up fresh from pure chemicals, all are subject to the disadvantage that on continued use at high temperatures some breakdown of the chlorides to oxides occurs. The resulting oxides have a definite decarburizing effect upon the steel, and accordingly it is essential and common practice to add rectifiers to such baths from time to time in order to maintain them in a. neutral condition. Among the rectifiers which have been used commercially are carbon, compounds of boron, and silicon or its compounds. Carbon operates to form a cover for the bath and hence to exclude atmospheric oxygen. Because it must necessarily be disturbed from time to time it does not provide complete protection and, in addition, it cannot be used satisfactorily for the work treated in baskets, due to the tendency to remove excess quantities of the carbon cover.

Boron compounds form complex borates with the oxides of the bath. These are insoluble. Some of them float on the surface and others sink to the bottom as a sludge which must be removed at frequent intervals. Those which float as a scum often stick to the work and present a diflicult cleaning problem.

Silicon and silicon carbide, as Well as silica, form a sludge which settles to the bottom of the pot and must be removed at regular intervals.

In accordance with the present method oonvennot be vaporized before they are injected into since it is relatively inexpensive, easy to obtain,

and relatively safe. Other equally satisfactory but slightly more expensive alkyl halide gases are dichlorodifiuoromethane and dichloromonofiuoromethane (Freon 12 and Freon 21, respectively).

The elements chlorine and fluorine are likewise each effective but are somewhat less desirable for commercial use because of their poisonous character which necessitates use of extreme care to avoid gas leaks. The same is true of anhydrous hydrochloric acid which is also effective as a neutralizer.

Iodine or iodine compounds cannot be used because iodine does not produce compounds with barium or sodium which are stable at heat treating temperatures.

Liquids such as bromine, carbon tetrachloride, or trichloroethylene are also efiective but they must be vaporized before they are injected into the bath, in order to prevent an explosive expansion. It is for this reason that materials which are gaseous at room temperature are preferred to the liquids.

The amount of rectifying material which must be injected in a. gaseous state depends upon the condition of the bath. A continuous injection of very small quantities is sufficient to maintain the bath in a neutral condition. If desired, the as may be injected intermittently from time to time as required by the condition of the bath. In no case should the amount be sufiicient to blow any of the molten salt out of the pot.

It is believed that the reactions which occur on the injection of methyl chloride are as follows:

First, the methyl chloride CH3C1 breaks down into hydrochloric acid and methylene or ethylene. The methylene or ethylene burn at the surface of the bath and the hydrochloric acid reacts with the barium oxide or sodium oxide, as follows:

From the above it is apparent that the hydrochloric acid, which results from the breakdown of the methyl chloride, will restore either barium oxide or sodium oxide to their original chloride for-m, thus leaving the bath in its initial state free from any addition agents. The water formed in the reaction passes off immediately as steam.

From the above reactions it is apparent how anhydrous hydrochloric acid likewise will rectify the bath. Since all primary halides break down into hydrochloric acid on heating they will result in similar reactions. In the case of chlorine and fluorine, which do not break down into hydrochloric acid, it is believed that a direct reaction results in a substitution of the halide for the oxygen in the barium or sodium oxide compounds in the bath.

No particular or unusual apparatus is required to carry out the process, exceptthat it is found that the use of a ceramic delivery tubefor the gas where it enters the heat treating potavoids plug;- ging of the tube by carbon.

While a number of elements or compounds have been specifically referred to, it is apparent that others of the broad class set forthi'n the specification may be employed within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. The method of rectifying a neutral molten salt heat treating bath. containing undesirable oxides which includes injecting ina gaseous state into the bath while it is in a heat treating pot a 4 methyl halide of the halide group consisting of chlorine, fluorine, bromine.

2. The method of rectifying a neutral molten salt heat treating loath containing undesirable oxides which includes injecting methyl chloride into the bath while it is in a heat treating pot.

PAUL H. KRAMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,241,113 Hayes Sept. 25, 1917 2,375,758 Bates May 15, 1945 FOREIGN PATENTS Number Country Date 494,274 Great Britain Oct. 24, 1938 OTHER REFERENCES Gregory: Uses and Applications of Chemicals and Related Materials, Reinhold Pub. Corp, N. Y. C., N. Y. (1939), page 315. 

